This section provides background information related to the present disclosure which is not necessarily prior art.
Internal combustion engines generally include a cylinder block defining one or more cylinders. A piston is situated in each of the cylinders for movement therein in response to the combustion of an air/fuel mixture contained within each of the cylinders. Internal combustion engines were traditionally designed to be naturally aspirated, meaning that the internal combustion engine would draw intake air from the environment in response to a low pressure created within each of the cylinders by the motion of the pistons. In an effort to increase the power output and efficiency of internal combustion engines of any given size (i.e., displacement), forced induction systems where developed.
One common type of forced induction system is called a supercharger. A supercharger generally includes a compressor that is disposed along an intake tract of the internal combustion engine. The compressor of the supercharger is mechanically coupled to an engine primary shaft such that the compressor is rotatably driven by the engine primary shaft. Often, this mechanical coupling is a belt and pulley system. The compressor uses the rotational energy of the engine primary shaft to increase the flow of intake air through an intake tract of the internal combustion engine. As a result, the pressure of the intake air entering the internal combustion engine is greater, allowing for more fuel to be added to the air/fuel mixture. This ultimately increases the power output of the internal combustion engine without changing its displacement.
When appropriately designed, supercharged engines can also be more fuel efficient when compared to naturally aspirated engines. Superchargers are especially beneficial because even at low engine operating speeds the compressor of the supercharger increases the pressure of the intake air resulting in instantaneous throttle response and a more linear increase in power output. Accordingly, superchargers do not suffer from any “lag” in power application.
Despite the foregoing benefits, superchargers have known disadvantages. First, superchargers increase the parasitic losses of an internal combustion engine because the internal combustion engine's own rotational power is used to drive the compressor. Such parasitic losses increase significantly at high engine operating speeds. Accordingly, some of the power output and efficiency gains associated with forced induction are cancelled out by the parasitic drag of the compressor. Second, the mechanical coupling between the engine primary shaft and the compressor transfers the torsional vibrations of the engine primary shaft to the compressor. Such torsional vibrations can decrease the performance and reliability of the compressor over time.
A second common type of forced induction system is called a turbocharger. Like a supercharger, the turbocharger also includes a compressor that is disposed along the intake tract of the internal combustion engine. Rotation of the compressor increases the flow of intake air through the intake tract, which increases the pressure of the intake air entering the internal combustion engine. Again, this allows for more fuel to be added to the air/fuel mixture and ultimately increases the power output of the internal combustion engine without changing its displacement. However, unlike with the supercharger, the compressor of the turbocharger is driven by a turbine that is propelled by exhaust gases produced by the internal combustion engine.
The turbine of the turbocharger is disposed along an exhaust tract of the internal combustion engine. The turbine generally includes a plurality of turbine vanes that rotationally drive the turbine in response to exhaust gas flowing through the exhaust tract. Accordingly, a turbocharger is most effective at high engine operating speeds where exhaust gas flow is great. When appropriately designed, turbocharged engines can also be more fuel efficient when compared to both naturally aspirated engines and supercharged engines.
Because the turbocharger is powered by the latent energy of the exhaust gases, the turbocharger does not increase the parasitic drag on the internal combustion engine like a supercharger does. Further, because there is no mechanical coupling between the engine primary shaft and the compressor, torsional vibrations of the engine primary shaft are not transmitted to the compressor of the turbocharger.
Despite the foregoing benefits, turbochargers also have known disadvantages. Increasing the flow of intake air to the internal combustion engine is dependent on the flow of exhaust gases expelled from the internal combustion engine. This leads to a phenomenon known as “turbo lag.” Turbo lag refers to delayed throttle response in a turbocharged engine. This characteristic occurs because there is a time delay between rapid throttle increases and an associated increase in exhaust gas flow. Accordingly, in a turbocharged engine there is a time delay between throttle increases and associated power increases of the internal combustion engine because the turbine must be allowed some time to “spool up” as exhaust gas flow increases. Another problem with turbocharged engines is that there are times where the compressor increases intake air flow beyond the requirements of the internal combustion engine. For example, this occurs when exhaust gas flow is high and the throttle is decreased. The excessive intake air pressure may be relieved from the intake tract by a device called a blow-off valve. The blow-off valve is connected in fluid communication with the intake tract and operates to release intake air from the intake tract when the pressure of the intake air exceeds a threshold value. A device called a wastegate may also be used to divert exhaust gases away from the turbine when the intake air flow generated by the compressor exceeds the requirements of the internal combustion engine. The blow-off of intake air and/or the bypassing of exhaust gases are losses that waste energy.